We measured daily urinary excretion rates of dopamine (DA) and dopa during dietary salt loading and natriuretic responses to exogenous DA in Dahl salt-sensitive (OS) and salt-resistant (DR) and Sprague-Dawley rats. Excretion rates of dopa increased by about 6-fold during salt loading in all rat strains. Maximal urinary dopa responses were attained within 1 day of salt loading. Daily excretion rates of DA also increased by about 5-6-fold in DS and DR rats and about 2-fold in Sprague-Dawley rats, with maximal DA responses attained by day 5. DA infusion (3 (mu)g/kg/min) increased urinary sodium excretion by 406% in Sprague-Dawley rats but only 267% and 147% in DS and DR rats (p<0.05 for Sprague-Dawley vs Dahl rats). The results demonstrate that salt loading markedly and rapidly increases dopa excretion in rats. Considering values for DA excretion in other rat strains, the results suggest that Dahl rats have increased formation of DA for a given amount of dopa delivery to the kidney and that this abnormality is unrelated to salt-sensitive hypertension in DS rats. The results also provide in vivo support for the view that the responsiveness of renal DA receptors mediating natriuresis is related to production of endogenous DA in the kidney.